Rate adaptive data broadcast technique

ABSTRACT

This invention describes a novel digital broadcast modulation scheme to provide wide area coverage at a high bit rate from a single transmitter. The scheme creates an additional dimension within which to create transmission channels besides those of time, code, direction and frequency. Advantages of scheme include lower power to deliver the same total bit rate to an area, wider coverage using the same power, and closer spacing of transmitters sharing the same frequencies. It is particularly well suited to supporting broadcast of data to a large population of users.

CROSS REFERENCE TO RELATED APPLICATIONS

U.S. Provisional Patent Application No. 60-465096

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO A MICROFICHE APPENDIX

Not Applicable

TECHNICAL FIELD

The technical field of the invention is data encoding and decoding forthe purpose of resilience in the face of noise in digital datacommunication systems.

BACKGROUND ART

With the advent of the Internet it has become increasingly popular totransmit or broadcast digital data over wireless links. The use of anycommunication medium is constrained by Shannon's law, which states thatthe amount of information that can be transmitted is proportional to thebandwidth available. There is a limited amount of spectrum availablethat does not have undesirable characteristics—such as line of sightrestrictions, or excessive attenuation in rain or fog. Typicalapproaches to reusing spectrum involve directionaltransmission/reception, use of low power cells, frequency hopping, orcoding. While it is theoretically possible to transmit more than 1bit/sec per Hertz of bandwidth (the unit is often abbreviated to “bitsper Hz”), in practice this is often close to the designed-in ratio formany wireless data systems. However, the desire for broadband (high bitrate) connections is pushing the industry into exploring ways toincrease this ratio and conserve on spectrum. For example, the IEEE802.11a standard prescribes 64 QAM as the modulation technique in highbandwidth mode, which is equivalent to 6 bits/sec/Hz. The ability toincrease this number is limited by the signal to noise (S/N) ratio atthe receiver, as this affects the receivers ability to distinguishbetween the different symbols in the symbol constellation. Therelationship is typically logarithmic, meaning that for every additionalbit/sec/Hz we have to double the S/N ratio. The effect on a wirelesssystem is to reduce coverage area, or increase the maximum requiredsignal strength.

Another effect of the use of digital transmissions is that there is avery sharp cutoff—upto a certain point noise has no effect on thesignal, but beyond a certain range the signal quickly becomes so garbledas to be completely useless. The sharpness of this cutoff is enhanced bythe fact that many digital transmissions are organized into messageblocks, and the whole block is discarded if any uncorrectable error isfound. For digital TV broadcasts this is considered desirable, youeither have a good picture or nothing. But for other types of uses thisis undesirable; people browsing the Internet, emergency personnel orpolice might be willing to live with a slower or lower qualityconnection, but being cutoff completely is disastrous.

There is also another effect with high bits/sec/Hz digitalsystems—because the signal is pretty high above the noise floor at thecutoff range, there is a large zone beyond the cutoff range wherereception at the same frequency of the transmitter is not possible. At20 bits/sec/Hz this zone can be 400 times the area of the zone wherereception is possible.

Compare this with an analog broadcast. As you get further and furtherfrom the transmitter, you lose clarity, but you can still recoversignificant amounts of information from the transmission. This does notrequire any coordination with the transmitter—it happens automatically.Many techniques have been implemented to adapt the rate of digitaltransmission to conform to a value suitable for adequate reception atthe receiver, but all rely on negotiation between the transmitter andreceiver. Moreover, while the low bitrate transmission is in progress,no other transmission can take place. For example, in 802.11a, onechanges the modulation technique from 64 QAM all the way down to BPSK asthe received SN ratio decreases.

REFERENCES

[1] U.S. Pat. No. 6,377,562 describes a primary application for thispatent, as well as another method of accomplishing some of theobjectives.

[2] U.S. Pat. No. 5,590,403 describes a system for communication betweena central network and mobile units.

[3] “The Capacity of Downlink Fading Channels with Variable Rate andPower” by Andrea Goldsmith published in IEEE Transactions in VehicularTechnology, Vol 46, No 3, August 1997 compares the capacity of variousmodulation techniques, and in particular shows the advantages of“superposition coding with successive decoding”.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide a encoding/decoding techniquethat allows digital broadcast transmissions to also have a failsoftcapability, i.e. as the amount of noise or range increases, the bit ratedecreases without completely losing the link. In particular, when usedin a wireless environment, nearby receivers get access to the full bitrate, while far away receivers get a bit rate compatible with the S/Nratio at the receiver. In addition, nearby receivers can continue to getdata while transmission to a far away receiver is in progress, losingonly the bit rate used for the remote receiver.

The key idea behind the invention is the fact that a fixed level ofnoise does not limit the receivers ability to distinguish between anypair of symbols in the constellation, only between some pairs. This, ifwe can find (or create) groups of symbols for which the receiver willstay within the group for certain levels of noise (i.e. it will not beconfused about which group the symbol belongs to), then we retain someinformation about what the original symbol was. If we can arrange thesegroups in a hierarchy of “non-confusable” groups for different noiselevels, then we can create a labelling system for each symbol where themost significant bits of the label are accurate for a particular levelof noise. If we then send messages using only bits of the samesignificance, then for a particular received S/N ratio, all messagesencoded with sufficiently significant bits will be accurate, while thosethe employ less significant ones will not. We will be able recover asignificant fraction of the messages, instead of losing practically allof them. In effect, the system creates an additional dimension withinwhich to create transmission channels besides the familiar ones of time,frequency, space, and code. Another way of thinking about the effect ofthe coding employed by this invention is that while the BER for anyparticular receiver for both conventional coding and this one are thesame, the message error rate is lower. This happens because weconcentrate all the bad bits into some subset of the messages, and sothe remaining messages still get through unaffected. This is the exactopposite of what IEEE 802.11 tries to do—they deliberately scramble theorder of the bits to prevent a long run of bad bits.

This technique can be applied to any modulation technique that has asymbol constellation of 4 or more symbols. What is required is that theeffect of standard noise on the receivers ability to distinguish betweeneach pair of symbols in the constellation be studied and mapped, andthen the symbols be organized into the hierarchical groups. It may benecessary to omit some symbols from the constellation allowed by themodulation technique to ensure that the groups are disjoint.

What distinguishes this system from other systems providing rateadaptive capability, is that the transmission of messages to receiverswith different received S/N ratios can happen simultaneously. In a databroadcast situation, a transmission to a receiver with a low S/N ratiodoes not block transmissions to receivers with higher S/N ratios, itmerely slows them down by the fraction of the available bit rate beingused by the low received S/N ratio transmission. Compare this with thesituation in 802.11a—a transmission to a far away receiver preventstransmission to nearby receivers; and since this is a slow transmission,the medium may be blocked for a while. During the transmission to a faraway receiver, only 1 bit/sec/Hz gets transmitted instead of 6. In thissystem, the far away receiver would get 1 bit/sec/Hz, while the nearbyreceiver would get only 5 bits/sec/Hz—but the transmitter has not sloweddown at all.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general diagram of the invention.

A multiplexor 1 takes 1 bit from each message and combines them into asymbol label. In some implementations, more than 1 bit per message couldbe combined into a symbol label. This is then converted into an analogsymbol using the DA converter 2 and then used to modulate a carrier inthe modulator 3. This is then transmitted to the demodulators 4, whichregenerate the analog symbol, and the label is regenerated by the ADconverter 5. The symbol label is fed to the demultiplexor 6, whichoutputs each bit of the label as the next bit of the correspondingmessage. When a message is complete it is transferred to the checker 7,which verifies (using techniques such as a checksum or MD5 hash) that ithas been received without errors. If so, the message may be acknowledgedvia a some back channel 8, and the message is passed to the ultimatereceiver 9. An optional SN ratio measuring circuit 10 providesadditional information to the checker so that it does not attempt toverify unusable channels. Note that the checking and acknowledgement arecurrently already performed by the IP subsystem of current computersystems.

DETAILED DESCRIPTION OF THE INVENTION

In the example embodiment, the modulation format considered will be AM(amplitude modulation), although the techniques can be obviously beapplied to other modulation formats. The signal to noise ratios for thismodulation format will be voltage based (rather than power based).

An eight bit word can label all of the symbols in 256 AM, which areessentially 256 equally spaced voltage levels. A classic 8-bit DAconverter can be used as the DA converter 2. Assuming perfect DA and ADconverters, if the signal to noise ratio is 256, then the receiver willconfuse only adjacent symbols. Imperfections in the converters can alsobe treated like noise. If the signal to noise ratio is 2 (3 db), thereceiver will confuse a symbol with symbols upto 127 levels away. Ineither case, since we cannot divide them into two non-confusable groups,we cannot be assured about the correctness of any bit in the label.

We now arrange that we will never send a symbol that has 1 in the secondmost significant bit of the label. The allowed symbol space then becomesdivided into two groups, one where the most significant bit of the labelis 1 and the other where the most significant bit is 0, with a large gapin between. If the signal to noise ratio is more than 2 (3 dB), thereceiver will always get the most significant bit of the label correct.Thus messages which are transmitted using only this bit will not haveerrors.

Note that in a normal 256 AM transmission each symbol would transmit 8bits of the message; what we are requiring here is that multiplemessages be sent simultaneously, with each symbol supplying oneadditional bit for each of the messages. In effect, we are creatingmultiple channels of transmission, with bit 0 of each symbol a part ofchannel 0, bit 1 of each symbol part of channel 1, etc. All the bits ofmessages in one channel have similar probabilities of being in error fora given SN ratio.

Let us now agree instead to not transmit the symbols with the labels 127& 128. Again, the receiver will always get the most significant bitcorrect—but only if the S/N ratio exceeds 128 (12 dB). Note that evenwithout the agreement, the accuracy of this bit would be in question forthis signal to noise ratio only if the receiver produced 127 or 128 asthe received symbol. In other words, if we had some way to estimate theSN ratio as being above 128 (12 dB), we could be confident about thevalue of this bit except when the receiver produced one of these twovalues. In fact, if the probability of these symbols being present inthe input stream is low enough, this bit will be accurate often enoughto get complete messages through the system, even if we do not deletethese symbols from the allowed symbol space.

What we did when we agreed to not transmit a 1 in the second mostsignificant bit was to divide the symbol space into two groups whichcould not be confused by the receiver in the presence of noise less thanhalf the signal. Similarly, what we did when we agreed to not transmitsymbols 127 & 128 was to divide the space into two non-confusable groupsfor noise less than 1/128 of the maximum signal.

Taking this a step further, look at what happens when we agree not totransmit messages using the second and fourth most significant bits ofthe label. For SN ratios of 2 (3 dB) or more the most significant bit isalways correct. For SN ratios of 8 (9 dB) or more the 3rd mostsignificant bit is also always correct.

We can tailor the ramp down by choosing which bits to omit. For example,we could omit use of every third bit of the label. Then we would keep 6bits, for SN ratios of 4 (6 dB), 32 (15 dB), and 256 (24 dB).

All of these are achievable with classical binary DA and AD converters.A ternary DA converter (one in which each bit is valued at 3 times theprevious bit) driven by a binary signal provides us with a built in gapbetween symbol spaces. This can be used to achieve a SN ramp of 3 (4.99dB) per bit.

If a high power standard symbol is broadcast repeatedly at regularintervals (such as the frame and line sync signals on a TV broadcast),the SN ratio can be estimated by a estimation circuit 10 and usablechannels identified. This signal can also be used to set the gain of thereceiver's AGC, just as a TV receiver would.

While such a signal would still be necessary to adjust the AGC, in factthe SN ratio estimation circuit is not strictly necessary. Techniquessuch as that used in PPP would delineate message boundaries. Generatingand transmitting a hash signature (or checksum) for each message andcomparing that with the computed hash signature (or checksum) of thereceived message can tell us with high probability whether the messagewas received correctly.

Obviously this can be extended to more than 8 bits per Hz—limited onlyby the precision with which the D-A and A-D conversion is accomplished.It can also be extended to modulation schemes other than AM, providedthe probability of confusion of different symbols by the receiver isunderstood and the symbols can be grouped into hierarchicalnon-confusable groups based on signal to noise ratio (or almost so basedon the probability of confusion).

This kind of system could be used in a digital broadcast situation, suchas for digital TV, video and other data distribution, or even highbandwidth Internet access—where the back channel is accomplished viaother means. The advantage of such a system is that instead of settingup multiple high bandwidth low power cell transmitters, one can set up asingle high power high bandwidth transmitter, and then allocate channelswithin the SN space to receivers. It is especially well suited tosupporting a situation where a large amount of data is periodicallybroadcast to and cached by a very large base of users, with varyinglevels of priority for updates for some data and users.

To give one an idea of the potential capabilities of such a system, astandard TV transmission is considered good if the SN ratio is about 60dB. Using AM, you need about 3 dB to generate a bit of information aboutthe power level of a symbol, thus you can get 20 bits/sec/Hz from astandard TV channel. Since a TV transmission uses about 6 MHz ofbandwidth, theoretically you should be able to get at least 120Mbits/sec within the protected area of a TV transmitter, if you can getyour DA and AD converters precise enough. Let us say the transmitter ispowered to the point where this performance is achieved upto a radius of0.5 km (approximately 1 KW in practice).

Let us now consider where the next transmitter using the same channel(and power) can be located. Its signal needs to be 60 dB below the firsttransmitter. Since distance based attenuation is about 6 dB for eachdoubling of range, the second transmitter needs to be at least 2**10times the 0.5 km, i.e. more than 500 km, away—or it would need to be outof the line of sight. If it were any closer, its transmissions wouldgenerate enough interference to cause a complete loss of data usingcurrent techniques of achieving 20 bits/sec/Hz. Secondly, beyond 0.5 km,the ambient noise would also cause a loss of data. In effect given thesetwo transmitters, there can be no reception using the same spectrum inthe region 0.5 km to 500 km from the transmitter.

However, upto 250 km away, at least 1 bit (the most significant bit) ofeach symbol would almost always be correct. Since in this invention,messages only use some of the bits of each symbol, any messages thatused only that one bit would be received correctly. Thus, 250 km away,we would still be able to get upto 6 Mbit/sec of digital data to thereceiver. Current systems recognize this problem and address it bychanging the data rate for near and far receivers, and prefixing a rateheader to let the receiver know what rate format is being used. Ineffect, sending the messages by the mechanism proposed hereautomatically turns the system into a 1 bit/sec/Hz system when the SNratio demands it. But in addition, one does not have to stoptransmitting the less significant bits, and one does not need the rateheader, so the same transmitter communicates more information to thesame area, with the same power.

Let us now consider what happens when we use a 1 W transmitter like theone allowed by the FCC for unlicensed transmissions in the 5.875 GhzU-NII band. The range for this is dependent on the ambient noise andterrain, but IEEE 802.11a specification considers −65 dbm received powerto be sufficient to receive 6 bits/sec/Hz, so −23 dbm should besufficient to receive 20 bits/sec/Hz. A 6 Ghz with standard dipoleantennas, this can be achieved when closer than about 2.8 m (8.5 ft). At28 m we can do 13 bits/sec/Hz, at 281 m we can do 6 bits/Hz, and at 2.8km we can do 1 bit/sec/Hz. Each 802.11a channel consists of 48 channelsrunning at 250K symbols/sec, for a total of 12M symbols/sec. Because ofthe need for error correction, and the limitation of 6 bits/sec/Hz, IEEE802.11a tops out at 54 Mbits/sec, and to achieve this rate the furthestreceiver would have to be less than 281 m away (6 bits/sec/Hz). Assumingthe system that uses every alternate bit, i.e. one usable bit every 6dB, this invention would simultaneously allow 12 Mbits/second upto 1.7km away, 12 Mbits/sec upto 850 m, 12 Mbits/sec upto 425 m, 12 Mbits/secupto 212 m, 12 Mbit/sec upto 106 m (total is now 60 Mbit/sec), and so onas we get closer.

This system is dependent on the quantization of the transmitted signal,with only the noise being allowed to have a continuous range of values.Thus its best application is when one transmitter is broadcastinginformation to multiple receivers—not when multiple transmitters arereceivable at one receiver. Of course if the transmitters do not operatesimultaneously and the AGC can react fast enough, or for each receiverall except one transmitter are sufficiently faint to be considerednoise, then it is still usable.

BEST MODE FOR CARRYING OUT THE INVENTION

In the preferred embodiment, an RF transmitter operating in a TV channelis fed a baseband signal from a 20 bit DA converter. This D-A converteris fed 20 bit words, and must produce an output that is precise to onepart in a million. Video cards today can easily accomplish this—thecurrent standard is pixels with 24 bit color on screens with 1024×768pixels, and can go as high as 2048×1024 pixels. One specifies the colorand intensity of each pixel with 8 bits each for red, green, and blue.On the receiver side, a high quality receiver feeds its baseband outputto a AD converter—such as a video capture card. Here again, 24 bit videocapture cards already exist, and will copy the digital output directlyto memory. This is now a system that copies (with some errors) thecontents of the video memory on the transmitter to the memory designatedto receive the video capture on the receiver.

One implementation could choose to stay within the NTSC B&W videolimits. On an NTSC system one would have approximately 15750 lines persecond, 14400 of which are usable. To stay within the video bandwidthlimit and to support the blanking interval each line will actually carryonly 100 data symbols—each of which is 20 bits deep. Since video cardsproduce upto 720 datapoints (pixels) per line, the code must compute thepixel values on the transmitter by interpolation, and similarly theoriginal symbols must be recovered—again by interpolation at thereceiver.

One now applies the techniques of the patent. Instead of treating eachsymbol as 20 bits of a message, each symbol is treated as thecombination of bits from upto 20 messages. If we were combining 20messages, each bit of the symbol would come from one bit of eachmessage. In the above mentioned NTSC based system, each frame of such asystem would support 9600 data blocks of 100 bits each. The total bitrate would be 28.8 Mbits/sec, with each channel getting about 1.44Mbits/sec. Messages intended for far away receivers would be assigned tothe bits in each symbol least susceptible to noise (for example, themost significant bit of each color), while messages intended for nearbyreceivers would be assigned to more susceptible bits. Any errordetection and correction bits would be computed for each messageseparately, and would use the same bits in their symbols as theirmessage. One can consider each bit of a symbol to be part of a differentchannel. Thus bit 0 of every symbol is part of channel 0, bit 1 of everysymbol is part of channel 1, bit 2 of every symbol is part of channel 2etc.

On the receive side each line is split up into 20 data blocks (orhowever many were prearranged). If necessary, corresponding data blockson multiple lines are combined to form messages. Each message isseparately checked for errors and thrown away if uncorrectable. Everynow and then the receiver informs the transmitter via some other medium(such as a dialup connection) of the error statistics on each channel,which allows the transmitter to select the optimum channel for eachoutgoing message.

This kind of system can even support fractional bit/Hz. One simplyallows the message to repeat at the same spot in the “frame”. If thereceiver gets too many errors on a channel, it sums frames with highcorrelations in that channel, and applies the technique again. Since thetransmitter knows which channels are having high error rates for aparticular receiver, or simply knows a priori which channels are likelyto have high error rates, it can simply retransmit messages to thatreceiver on multiple consecutive frames on that channel. On a regularanalog TV system, ghosts due to multipath refections are not eliminatedby such an averaging process; however, since each channel carriesuncorrelated messages and the higher channels have less repeats, on thissystem ghosts would also tend to get suppressed.

This embodiment is regarded as best only in that it can be built quicklyusing off-the shelf parts. A custom system could expand the bandwidthallocated to the video intensity, and omit all other details of the TVsystem except for the frame synchronization mark. Other systems coulduse modulation other than AM, provided the effect of noise on theprobability of a receiver confusing pairs of symbols in theconstellation is understood.

1) A mechanism to simultaneously transmit multiple messages in a digitalbroadcast system, comprising; a multiplexor to combine one or more bitsfrom multiple messages into a digital symbol, so each symbol has bitsfrom multiple messages; a DA converter, which could be comprised of aseparate DA converter and an RF modulator, that converts the digitalsymbol into an analog symbol; an AD converter, which could be comprisedof a separate RF demodulator and AD converter, that converts the analogsymbol back into a digital symbol; a demultiplexor that recreates eachmessage from corresponding parts of multiple digital symbols. 2) Toincrease the probability of a whole message being successfullytransmitted, the mechanism of claim 1; where the symbols transmitted bythe system between the multiplexor and demultiplexor are selected from ahierarchy of non-confusable groups; where the symbols are labelled suchthat each portion of the label makes a choice at a particular level ofthe hierarchy; and each message is transmitted using only the portion ofthe symbol label for each symbol that makes a choice at the same levelof the hierarchy. 3) To increase the probability of a whole messagebeing successfully transmitted, the method of claim 2; where someportions of the symbol label are deliberately left unused, acting asguard bands to prevent messages using the less significant bits frominterfering with messages using the more significant bits. 4) Toincrease the total data rate successfully transmitted, the method ofclaim 3; where different receivers are grouped into groups withdifferent received S/N ratios; where transmissions to receivers ingroups with lower S/N ratios are done by using the more significant bitsand transmissions to receivers in groups with higher S/N ratios are doneby using the less significant bits. 5) To increase the probability ofhigh priority messages being received correctly, the method of claim 3;where higher priority messages are sent using the more significant bits,and lower priority messages are sent using the less significant bits. 6)The means of claim 2 or 3; where a computer determines the channel to beused by a transmitted message, and adds information to each message toallow an error check to be performed; where another computer determineswhether a message was received correctly, and maintains statistics as tothe error rates for each channel used to transmit messages, andperiodically communicates all or some of these statistics to the firstcomputer; and the first computer makes its selection at least partiallybased an the information provided by the second computer. 7) A method toextend the range of the system of claim 2 or 3 even further; where eachmessage is transmitted within frames delineated by at least onesynchronization mark; where messages not acknowledged as being correctlyreceived are retransmitted in the same spot of subsequent frames; wheremessages not correctly received are added to a frame channel buffer; andwhere the equally significant bits of the frame channel buffer arecombined to form another message. 8) The means of claim 1; where the DAconverter is a video graphics card; and where the AD converter is avideo capture card. 9) The means of claim 8; where the data sent isspread over the pixels to be displayed by the video graphics card bysoftware in the computer containing the video graphics card so that theoutput signal stays within the NTSC limits; and where software in thecomputer containing the video capture card recreates the data sent byinterpolating the pixels received by the video capture card.